1. Field of the Invention
The present invention relates to a liquid crystal display device which can be used, for example, for a TV screen, various OA instruments and a display panel of automobile, and a method for preparing the liquid crystal display device.
2. Description of Related Art
Hitherto, a liquid crystal display device has been formed by injecting a liquid crystal material between a pair of transparent electrodes which are fixed with a gap of several micrometers. However, the above structure can not give the preparation of the display having a large area. In addition, brightness of the screen and an angle of view field are insufficient, since it is necessary to attach polarization plates having polarization axes which are perpendicular to each other to a pair of glass substrates enclosing the liquid crystal material.
Recently a new liquid crystal display device has been developed by Prof. Kajiyama et al of Kyusyu University, Japan [cf. for example, Polymer Preprints, Japan Vol. 37, No. 8, 2450 (1988); Chemistry Letters, 813-816 (1989); Chemistry Letters, 679-682 (1979); and Journal of Applied Polymer Science, Vol. 29, 3955-3964(1984)]. This device is prepared by casting and coating a solution of a polymer and a liquid crystal material in a solvent on a transparent electrode, evaporating the solvent to separate a polymer phase from a liquid crystal phase so as to form a composite film in which continuous pores in a polymer matrix having a three-dimensional network structure are filled with the liquid crystal material, then positioning another transparent electrode on the composite film.
In the above liquid crystal display device, when no voltage is applied, the incident light is scattered and the composite film is opaque since liquid crystal molecules in the pores are in random state. When a voltage is applied between a pair of transparent electrodes which sandwich the composite film, the liquid crystal molecules orient in the direction of electric field through the electro-optical effect, the incident light passes through the composite film without scattering and the composite film is converted to a transparent state in the case of EQU .DELTA..epsilon.&gt;0
wherein .DELTA..epsilon. is an anisotropy of a dielectric constant and defined by the equation: EQU .DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp.
(wherein .epsilon..sub..parallel. is a dielectric constant in the direction of the molecular axis and .epsilon..sub..perp. is a dielectric constant in the direction perpendicular to the molecular axis).
The liquid crystal display device having the above structure can easily be made with a large area since the above composite film having the electro-optical effect can be prepared only by coating and drying a solution containing the polymer and the liquid crystal material. Since the composite film has flexibility by selecting the polymer and a flexible transparent film having electrical conductivity by the formation of a transparent electrically conductive layer on a surface can be used as the transparent electrode, the liquid crystal display device advantageously has flexibility.
The characteristic in the preparation of the above composite film is that the three-dimensional network structure is formed by the phase separation because of incompatibility of the polymer with the liquid crystal material when the solvent evaporates after casting the homogeneous solution containing the polymer and the liquid crystal material on the transparent electrode. Namely, the phase separation is induced by the solvent evaporation.
A method for preparing such composite film is known from the above literatures.
There are several known methods for preparing the composite film which contains the polymer and the liquid crystal and exhibits the electro-optical effect having the transformation between transparent and opaque states.
For example, H. G. Craighead et al., Appl. Phys. Lett., 40 (1), 22 (1982) and U.S. Pat. No. 4,411,495 disclose a method for filling the pores of an already formed porous polymer film with a liquid crystal material. According to this method, the polymer is separated from the liquid crystal material from the beginning and a step of the phase separation is not included.
Japanese Patent Kohyo Publication No. 501631/1983 (J. L. Fergason) discloses a method which comprises forming microcapsules from a liquid crystal material in an aqueous solution of polyvinyl alcohol to prepare a dispersion and then coating the dispersion.
In this method, when the liquid crystal material forms the microcapsules, the liquid crystal phase is separated from the polyvinyl alcohol phase. The solvent, namely water is a medium used for only the coating, and the evaporation of water does not participate in the phase separation. In the resultant film, the liquid crystal material is present in the form of a droplet covered with a capsule.
Japanese Patent Kohyo Publication No. 502128/1986 (J. W. Doane) discloses a method for thermosetting a mixture of an epoxy resin and a liquid crystal material with a curing agent. In this method, a solvent is absent and a phase separation is induced by the formation of a high molecular weight material through the curing of the epoxy resin. A liquid crystal material is present in the form of droplets in the resultant film.
Japanese Patent Kokai Publication No. 62615/1989 discloses a method for photosetting a mixture of a photosetting compound and a liquid material with the light exposure. Also in this method, a solvent is absent and the phase separation is induced by the formation of a high molecular weight material through the curing of the photosetting compound. Polymer Preprints, Japan, 38 (7), 2154 (1989) describes that the liquid crystal material is dispersed in the form of droplets in the film.
"The phase separation by the solvent evaporation" is an original method which is entirely different from the above other methods and was firstly published by the above literatures of Kajiyama et al. The liquid crystal material is present in the form of a continuous phase (not in the form of droplets) in the continuous pores of a polymer matrix having a three-dimensional network in the film. This is also distinct characteristics of the composite film of Kajiyama et al.
However, the liquid crystal display device having the composite film of Kajiyama et al has insufficient heat resistance. When the liquid crystal display device is used in a place continuously exposed to a high temperature, such as a display panel of automobiles, the contrast between the transparent and opaque states decreases, and the original contrast can not recovered even if the temperature of the device is returned to a room temperature.
It was found that since the conventional liquid crystal display device uses a thermoplastic resin such as an acrylic resin and a methacrylic resin as the polymer matrix, the three-dimensional network of the polymer matrix collapses and the contrast of the device accordingly decreases when the device is exposed to a high temperature.
To solve the above problem, it may be contemplated to form the polymer matrix from a hardening resin such as a thermosetting resin and a photosetting resin. The hardening resin has a three-dimensional network molecular structure which hardly thermally deforms. In addition, the hardening resin has a larger molecular weight than the thermoplastic resin and good heat resistance.
However, since the hardening resin which is cured to form the three-dimensional network molecular structure is hardly dissolved in a solvent, the phase separation method as stated above cannot be employed and the composite film cannot be prepared.